In this study we will take advantage of the ease of manipulation of yeast for the study of the regulation of phospholipid synthesis in relation especially to mitochondrial biogenesis: (a) Synchronized yeast phospholipid biosynthesis, including cardiolipin biosynthesis is periodic as is mitochondrial DNA synthesis, while mitochondrial cytochrome synthesis is continous through the cell cycle. (b) The regulation and isolation of two enzymes of phospholipid synthesis are being studied. The enzyme CDP choline diglyceride phosphorylcholine transferase is probably located in the plasma membrane and/or endoplasmic reticulum, in contradistinction to our earlier claim that it was in the outer mitochondrial membrane. The enzyme activity varies through the cell in periodic fashion, and is unaffected by choline variations in the nutrient medium. We have had preliminary success in solubilizing this enzyme. The second enzyme, CTP phosphatidic acid cytidyl transferase, involved in the formation of CDP diglyceride, a precursor of mitochondrial cardiolipin amongst other acidic phospholipids, is situated predominantly in the inner mitochondrial membrane. It too has been solubilized, stabilized and purification is being attempted.